Apparently I am a total failure ..
.. as a Cat Staff person - I have that on good authority.
Bought a different, and cheaper, brand of cat food earlier this week. Both of them ate it - seemed to enjoy it - then all hell broke loose.
Willow was constipated - and cried when she tried to defaecate - bought medicine on advice from from vet - and her bottom exploded - everything she has ever eaten for at least a year - jet propelled all over the kitty box and the loo floor!
Meanwhile ...
Nemo began projectile vomiting - everywhere!
Vet has advised starving both of them for 24 hours as long as they have access to plenty of boiled/cooled water.
They do not like being starved. They do not like boiled/cooled water - and are protesting - LOUDLY - about it.
I am sure the neighbours must be thinking I am strangling them and I expect a visit from Cat's Protection inspectors any minute - they are just round the corner!!
Moral of the story - stay away from cheap cat food!